The invention particularly relates to the handling of hot melts and other materials that are normally in solid form at room temperature and must be heated in order to obtain fluid properties therefor. The invention is specifically directed to a method and apparatus for the coating of a web with a hot melt material.
There are numerous proposals in the prior art for handling hot melts and for coating webs with hot melts; exemplary prior art structures are the Park Hot Melt Coater, the Kroenert Pak 400 and Pak-Melt Coaters, and structure illustrated in U.S. Pat. No. 3,938,924. While such structures are generally useful for performing their intended functions, they normally have numerous disadvantages associated therewith. For instance, such prior art devices normally use mechanical devices to stir, agitate or otherwise mix the material to insure uniform temperature and viscosity, and this mechanical agitation results in shearing of the hot melt and destruction of the integrity thereof. Additionally, such prior art structures normally maintain a larger volume of hot melt at application temperature than is efficient, with resulting thermal degradation. The prior art also requires generally more complicated structures including positive displacement pumping means and the like, and normally mechanically pumps material through the filter to process, and uses deaerators to remove air from the material. Additionally, often times there are numerous atmospheric exposure points in the prior art apparatus which provide an opportunity for degradation of the hot melt by oxidation, or require a supply of inert gas over large volumes in order to prevent oxidation. Additionally, much material is normally wasted during start-up of such prior art structures, and thermal degradation and oxidation and waste in general exist at the point of application when the structure is idle or during start-up and shut-down.
According to the apparatus of the present invention all of the above-mentioned drawbacks inherent to one extent or the other in the prior art are eliminated. According to the present invention a simple method and apparatus are provided for the coating of a web with hot melt material or the like. The apparatus according to the present invention is much simpler than the prior art, eliminating the positive displacement pumps in the prior art, mechanical agitators, deaerators, and the like without consequent elimination of their functions. The filter pressure and the supply pressure at the extrusion head are isolated, and a very small volume (i.e., 1 gallon) of hot melt need be maintained at application temperature at one time. Additionally, the areas of atmospheric exposure are limited, and such areas are provided so that an inert blanket is not necessary, or that if an inert blanket were provided, only a minimum amount of inert gas need be required. Additionally, the hot melt that is supplied at the extrusion head has generally less thermal degradation, oxidation, and mechanical shearing destruction, and is substantially air free so that undesired bubbles are not formed on the coated product.
According to the apparatus of the present invention a web is coated with hot melt material by feeding the hot melt material from a source to a filter and to a tank, supplying hot melt from the tank to an extrusion head, passing the web to be coated in operative relationship with the extrusion head to coat the web with hot melt, isolating the source pressure from the supply pressure at the extrusion head, and pumping the hot melt from the tank to the extrusion head solely by providing pressurized gas at slightly more than one atmosphere pressure over the hot melt in the tank. The pressurized gas may be air at about 15 psi. The hot melt is fed into and withdrawn from the tank in such a manner that bubbles in the hot melt come to the surface and pop and/or form a foam on the top of the hot melt so that the withdrawn hot melt is substantially air free yet an inert gas blanket need not be provided. However, inert gas may be provided at about 15 psi as the pressurized gas in the tank. The method may further comprise the step of metering the hot melt to a thin film flowing over heated surfaces in feeding the hot melt from the source to the tank, and effecting turbulent flow from the tank to the head, and thereby insuring thermal uniformity without agitation of the hot melt. The level of hot melt in the tank is maintained at a predetermined level, and the extrusion head is maintained substantially at that predetermined level also so that the extrusion head need never be drained and start-up waste of hot melt is substantially reduced.
Apparatus according to the present invention comprises apparatus for supplying liquid material to coat a web comprising a closed system including a source of liquid material, filtering means, a tank, an extrusion head, means for supplying liquid material from the source to the filtering means, means for supplying liquid material from the filtering means to the tank for isolating the material pressure at the source from the pressure at the extrusion head, means for pumping the liquid material from the tank to the extrusion head (said pumping means consisting of means for providing gas at slightly more than one atmosphere pressure over the material in the tank), and means for transporting the pumped liquid to the extrusion head from the tank. Means are also provided for maintaining liquid material in the tank at a predetermined level, the predetermined level being maintained at the level of liquid in the extrusion head so that upon venting the pressurized gas from the tank the extrusion head will not drain or overflow. The volume of the tank may be as low as 1 gallon.
The apparatus according to the present invention further comprises apparatus for supplying liquid material for application to an article including an extrusion head, a tank operatively connected to the extrusion head, means for pumping liquid material from the tank to the extrusion head (said pumping means consisting of means for providing gas at slightly more than one atmosphere pressure over the material in the tank), and means for maintaining the liquid level in the tank at a predetermined level, the predetermined level being maintained at the level of liquid in the extrusion head so that upon venting of pressurized gas from the tank the extrusion head will not drain or overflow.
It is the primary object of the present invention to provide an improved apparatus for the application of liquid materials to a web, especially hot melts. These and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.